Looks like some genuine file system corruption. For some reason, most flash file system developers seem to believe file system corruption is impossible, thus don't offer tools to check and repair them. UBIFS seems to be similar.

I would suggest to back everything up using a file-based backup program (tar, cpio, ...), reflash the whole plug and restore the [changed] files from the backup.

After the backup and before reflashing, you may want to play with the various debug options in case they give you some hints on how to possibly repair the file system. Or send a bug report. More information can be found in the Linux source tree under Documentation/filesystems/ubifs.txt (or someplace on the Internet like here: http://www.mjmwired.net/kernel/Documentation/filesystems/ubifs.txt